Flexible packaging materials comprising layers of thermoplastic polymers are used to package a variety of products, including food. For example, cereals, potato chips, and other snack foods are packaged in such materials. The packages are formed in packaging machines in which the material is sealed to itself by heated sealing jaws. However, when a package is opened by pulling apart one of the heat seals, normally at the top of the package, the heat seal is not resealable without the application of heat as well as pressure.
This may be a problem for the consumer who does not wish to use the entire contents of the package at one time. After having opened the container in order to remove part of the contents, the consumer may want to be able to reseal the container easily and effectively. For example, in the case of foodstuffs, the consumer may want to be able to reseal the package containing the unconsumed foodstuff in order to maintain its freshness or palatability. Thus, there is a need for a sealing structure for containers which allows easy opening and effective and simple resealing of the container after part of its contents has been removed.
Resealable or reclosable packages have been developed to meet this need. In this type of package, the adhesive plays an essential role insofar as, once the package has been opened by the end user, the quality of the reclosure and of the reopening will depend on the performance of the adhesive. Pressure sensitive adhesives remain tacky at ambient temperatures and can be adhered to substrates or structure layers simply by manual pressure. This property means that the PSA will stick to many surfaces unless it is masked during processing to make and fill a package.
Reclosable packages may incorporate a multilayer film in which a pressure-sensitive adhesive (PSA) is positioned between a layer of a heat-sealable material and a substrate or structure layer.
To prepare a reclosable package, a portion of the heat-sealable layer of the film is contacted to a container or to another portion of the heat sealable layer and heat-sealed to it. When the package is first opened, the sealant material is torn in the area of the heat seal, thereby exposing the PSA. The package is reclosable by the application of pressure alone to the PSA, which adheres to the package in the former heat-seal area.
For example, EP0160975 discloses a hot- or cold-sealable multilayer sheet for reclosable packages. U.S. Pat. No. 5,089,320 relates to a multilayer film for reclosable packaging obtained by coextrusion or coextrusion coating.
GB2319746 discloses two films, one serving as a container after thermoforming, the other serving as a lid intended to be heat-sealed around the outline of the first film, and containing a layer of PSA-type adhesive consisting of an elastomer and a tackifying resin. The sealing layer for the two films can be an ionomer, for example available under the tradename SURLYN® from E. I. du Pont de Nemours & Company, Wilmington, Del. (DuPont).
U.S. Pat. Nos. 6,302,290, 6,511,723, and 6,777,050 disclose films and packages suitable for reclosable packages. Some of the films are obtained by blown film coextrusion with the PSA as the inside layer of the bubble and an outer layer that is suitable as a heat-sealable layer, followed by collapsing the bubble to encapsulate the PSA between two layers of the heat-sealable material. This process necessarily limits the film to symmetrical structures or requires additional steps to laminate the symmetrical structure to other layers to provide a suitable packaging film. A film comprising the structure polyolefin/PSA/polyolefin prepared by a collapsing bubble technique may be adhesively laminated to biaxially oriented polyethylene terephthalate (boPET) or biaxially oriented polypropylene (boPP).
Previous films for reclosable packages have been limited to those in which the sealant layer comprises polyolefins (e.g., low density polyethylene (LDPE) or high density polyethylene (HDPE) or metallocene polyethylene (mPE; PE produced by metallocene catalysis)), or ionomers. These materials have been used because they can be coextruded easily with the PSA because of their melt processing temperature and because they are the most commonly used seal layers in packaging.
However, use of polyolefins has drawbacks, including limited adhesion to other packaging materials such as polyethylene terephthalate (PET). They may also exhibit stringing on first opening the package because of their high elongation at break. Stringing is the formation of long filaments of the sealant layer that are pulled from the seal interface as the film is peeled away.
Amorphous polyethylene terephthalate (APET) as a heat seal layer has good adhesion to PET. However, coextrusion of a PSA in contact with APET is very challenging because the minimum melt processing temperature of APET is above 250° C. (a typical APET melts at around 225° C.) and the maximum melt processing temperature of the PSA to avoid degradation is 180° C.
Reclosable lids for sealing to PET have been produced by first coating the PSA on boPET, then applying an overcoating to avoid blocking in the roll. In a second step the structure is coextrusion-coated with tie/APET but the tie resin has high elongation at break. Also, the cost of these films is high because of the two steps necessary to prepare the film.
U.S. Pat. No. 7,422,782 claims a multilayer film suitable for use in packaging applications comprising at least a polymeric first layer having a first surface and an opposing second surface wherein the polymeric first layer comprises a heat-sealable, water-insoluble aliphatic polyester; a polymeric second layer having a first surface and an opposing second surface and comprising a pressure-sensitive adhesive, wherein the second surface of the polymeric second layer is in contact with the first surface of the polymeric first layer and forms a peelable/resealable bond therebetween; and a third layer in contact with the first surface of the polymeric second layer. Because aliphatic polyester such as polylactic acid (PLA) has a melting temperature below 180° C., it can be coextruded at melt temperatures of less than 200° C., just above the maximum melt processing temperature of PSA.
U.S. Pat. No. 7,622,176 discloses a hot-extrudable PSA suited for reclosable packages and multilayer films in which the hot-extrudable PSA is positioned between a separable and sealable layer and a complexable layer. In the films, the sealable layer and the complexable layer are limited to materials not including polyester. The three-layer film may be laminated to a nonsealable film to give the final package mechanical properties, barrier properties, printing properties, etc.
Accordingly, there exists a need for films useful in reclosable packages with improved properties such as reduced elongation at break and/or better adhesion to packaging materials such as PET. Desirably, they are prepared by coextrusion processes that involve only a single step.